Cambodia's main political parties say they are closer to an agreement following talks between Prime Minister Hun Sen and opposition leader Sam Rainsy.

The two sides to set up a committee for election reform and to ensure that future protests are peaceful. But the two sides are still unable to agree on an opposition demand for an independent investigation into the disputed election.

The Cambodia opposition says it is holding firm to a threat to boycott the September 23 opening session of parliament, potentially stalling the formation of a new government if a political solution is not found before then.

After the meeting, Sam Rainsy promised his supporters that he will not betray their will in his negotiations with the government.

"We will not attend the first parliamentary opening, unless we receive a proper solution that represents the people's will. So long as there is no proper solution, we'll keep demanding it in order to render justice to voters," he said.

Tuesday's meeting was accompanied by a peaceful opposition protest of more than 10,000 in the Cambodian capital.

On Sunday, protesters marched through Phnom Penh and clashed with police who were armed with tear gas, smoke grenades and water cannons. Human rights activists say one person was fatally shot on an overpass leading to Freedom Park, where 20,000 opposition activists had gathered for a rally addressed by Rainsy.

A political crisis has gripped Cambodia since the election commission certified Hun Sen's Cambodian People's Party as the winner of the July election with 68 seats in the national assembly, ahead of Rainsy's CNRP with 55 seats.

The CNRP has rejected those results, alleging widespread fraud.

Hun Sen has led Cambodia for 28 years and is seen by critics as an authoritarian ruler despite overseeing a period of prolonged economic growth. Sam Rainsy was not a candidate in the election, having been in self-imposed exile before returning to Cambodia in July. He returned home after being pardoned by the king for criminal convictions which Rainsy says were politically motivated.